Power devices are widely used to carry large currents and support high voltages. Conventional power devices are generally fabricated from monocrystalline silicon semiconductor material. One type of power device is the bipolar junction transistor (BJT). The BJT is a well-known and frequently used power device. A BJT typically includes a semiconductor material having two opposing p-n junctions in close proximity to one another, thus, BJTs may be referred to as “n-p-n” or “p-n-p” transistors.
In operation, current carriers enter a region of the semiconductor material of a first conductivity type adjacent one of the p-n junctions, which is called the emitter. Most of the charge carriers exit the device from a region of the semiconductor material of the first conductivity type adjacent the other p-n junction, which is called the collector. A small portion of semiconductor material known as the base, having a second conductivity type, opposite the first conductivity type of the collector and the emitter, is positioned between the collector and the emitter. The two p-n junctions of the BJT are formed where the collector meets the base and where the base meets the emitter.
When current is injected into or extracted from the base, depending upon whether the BJT is n-p-n or p-n-p, the flow of charge carriers, i.e., electrons or holes, which can move from the emitter to the collector, may be affected. Typically, small currents applied to the base can control proportionally larger currents passing through the BJT, giving it usefulness as a component of electronic circuits. Structural and operational details of BJT's are discussed in Solid State Electronic Devices by B. Streetman (2nd edition (1980), chapter 7), the content of which is incorporated herein by reference as if set forth in its entirety.
The material that makes up a device contributes to the operability and usefulness of the device. For example, conventional BJTs are typically formed of silicon (Si), but can also include gallium arsenide (GaAs) and indium phosphide (InP). Recently, silicon carbide (SiC) has been used as a material for BJTs. SiC has advantageous semiconductor characteristics, for example, a wide bandgap, high electric field breakdown strength, high electron mobility, high thermal conductivity, high melting point, and high saturated electron drift velocity. Thus, relative to devices formed in other semiconductor materials, for example, Si, electronic devices formed in SiC may have the capability of operating at higher temperatures, at high power densities, at higher speeds, at higher power levels and even under high radiation densities.
Due to their ability to function at high frequencies, high temperatures, and high power levels, SiC transistors are highly desirable for use in applications such as high frequency transmitters for radar and communications, for high power switching applications, and for high temperature operations such as jet engine control. Accordingly, methods of producing device quality SiC and devices formed from SiC have been of interest to scientists and engineers for several decades.
SiC crystallizes in over 150 different polytypes, or crystal structures, of which the most common are designated 3C, 4H, and 6H where “C” stands for “cubic” and “H” for “hexagonal.” SiC is a difficult material to use in semiconductor devices. In particular, the high melting point of SiC may render techniques, such as alloying and diffusion of dopants, more difficult, usually because a number of the other materials necessary to perform such operations tend to break down at the high temperatures used to affect SiC. SiC is also an extremely hard material, and indeed its most common use is as an abrasive. Attempts have been made, with some success, in manufacturing junctions, diodes, transistors, and other devices from SiC.
SiC BJTs are discussed in, for example, U.S. Pat. No. 4,945,394 to Palmour et al., the disclosure of which is incorporated herein by reference as if set forth in its entirety. As discussed therein, SiC BJTs are formed having a base and an emitter formed as wells using high temperature ion implantation. Furthermore, SiC BJTs are also discussed in U.S. Pat. No. 6,218,254 to Singh et al., the content of which is incorporated herein by reference as if set forth in its entirety. As discussed therein, SiC BJTs having a base and an emitter are formed using epitaxial growth and high temperature ion implantation. Singh further discusses the use of removable dielectric spacers to improve the alignment of base and emitter regions.
SiC BJTs typically include at least one highly doped implant region for the base or the emitter. The formation of the implant region may cause damage to the SiC crystal. For example, the current gain of the device may be reduced due to surface recombination current due to implant induced defects. The current gain of the device may be reduced as a function of time due to stacking faults in the base region, which may result from the defects created by the implantation process. Furthermore, the highly doped emitter regions present in these devices may have a poor lifetime, which may further reduce the current gain of the device.
Referring now to FIG. 1, a cross section of a conventional SiC BJT will be discussed. As illustrated in FIG. 1, an N− SiC layer 11 is provided on a highly doped N+ SiC substrate 10. A p-type (P) SiC layer 12 is provided on the N− SiC layer 11. As further illustrated in FIG. 1, n-type emitter regions 20 (N+) and highly doped p-type base regions 25 (P+) are provided on the p-type layer 12. The emitter regions 20 may be formed by epitaxial growth and subsequent etching and base regions 25 may be formed by implantation techniques. In particular, the implantation of the P+ base regions 25 may be carried out at from about 650° C. to about 1100° C. using aluminum ions (Al+) having energy ranging from about 20 keV to about 360 keV. As discussed above, this implant may cause damage to the SiC crystal, which may not be entirely removed by a high temperature (1600-1700° C.) activation anneal.
In particular, the defects introduced by the implant may result in surface recombination current and stacking fault formation in the base region of the device. This relationship is represented by the equation:β=iC/iB where β represents current gain, ic represents collector current and iB represents base current. Base current iB may be further represented byiB=iBr+iBi+iBsr;where iBr is recombination current, iBi represents majority carriers injected across the emitter junction, and iBsr represents surface recombination current. Current gain may be represented by the equation:β=iC/(iBr+iBi+iBsr).Accordingly, any increase in surface recombination current iBsr in the base region yields a lower current gain for the device.
Referring again to FIG. 1, a passivation layer 18 is provided on the surface of the device. The BJT further includes an ohmic metal 30/31 and an overlayer metal 16 that provide contacts to the base (B) regions 25, the emitter (E) regions 20 and the collector (C) regions 10/11. The highly doped regions designated as N+ 20, 10 and P+ 25 may be provided to improve the quality of the ohmic contacts.